Add 'location list' subcommand & support --location on 'server create'.

This command adds the following subcommand:

    knife bluebox location list

A `--location` flag is also added to `knife bluebox server create` and
will default to the Seattle location.
Merge pull request #28 from fnichol/location-support

Add 'location list' subcommand & support --location on 'server create'.